Biosensors such as sensor proteins are molecular sensors obtained by connecting molecular recognition mechanism and signal transduction technology. In order to construct a sensor protein by using highly specific molecular recognition ability of protein, it is necessary to link two sites, i.e., a binding site for recognizing a target substance and a reporter site for generating a signal.
There have been reported some examples of sensor proteins designed by means of protein engineering (Hellinga & Marvin 1998. Trends Biotechnol. 16, 183–189). For example, there have been known a cAMP sensor which uses the character that cAMP binds to a cAMP-dependent protein kinase to dissociate two subunits of the cAMP-dependent protein kinase (Adams et al. 1991. Nature 349, 694–697); a maltose sensor using the character that when maltose binds to a maltose-binding protein, the structure of the maltose-binding protein changes (Marvin et al. 1997. Proc. Natl. Acad. Sci. USA 94, 4366–4371); a calcium ion sensor using the structural change of calmodulin (Miyawaki et al. 1997. Nature 388, 882–887; Romoser et al. 1997. J. Biol. Chem. 272, 13270–13274) and the like.